Quantum information processing promises to perform some significant tasks far more efficiently than can be accomplished classically. Important examples of possible applications include quantum computation, quantum simulation, and quantum communication (collectively, “quantum information processing”). Physical systems of various kinds are under consideration for quantum information processing. Trapped atomic systems, which may be ions or neutral atoms, constitute one promising type of physical system.
A common characteristic of physical environments for quantum information processing is that they can support qubits and that they can maintain the qubits for a coherence time that is long enough to permit quantum computations to take place.
A qubit is a physical system that has two quantum mechanical states, and that can exist in a superposition of those two states. The possibility of superposition of states is an essential feature of quantum information processing. The two states of a qubit are often represented in Dirac notation by the symbols 10> and 1>, respectively.
Individual qubits are defined in the trapped atomic system by isolating two quantized energy levels of the atomic configuration. Different states can include configurations of various properties of the atomic electron and nucleus such as electron orbit, electron spin, and nuclear spin. Controlled atomic transitions can be performed by applying excitation pulses at suitable resonant energies. Although these pulses are typically pulses of electromagnetic radiation, it has also been proposed to use mechanical energy, in the form of phononic pulses, to induce transitions in some systems.
Another important feature in many aspects of quantum information processing is entanglement. Two particles are said to be entangled if the quantum state of one cannot be described without reference to the other. Stated more formally, a system is entangled if its quantum state cannot be factored as a product of the individual states of its constituent particles. As a consequence of entanglement, the outcome of an experiment that collapses the quantum state of a first particle to produce an observable measurement can be correlated with the outcome of a similar experiment performed on a second particle that is entangled with the first, even if at the time of measurement the particles are separated by a macroscopic distance that precludes mutual interaction.
Quantum computing fundamentally depends on the ability to concurrently entangle and control a large number of qubits. However, the technical barriers to scaling quantum computing to large numbers of entangled qubits are high. For example, inhomogeneity has been an impediment in systems based on quantum dots, spectral crowding has been an impediment in systems that rely on proximity-based entanglement in ions, weak interactions have been an impediment in systems using neutral atoms, and extreme requirements for fabrication tolerances have been an impediment in systems based on silicon vacancies and on SQUIDs.
Accordingly, there is a need for new physical systems for quantum information processing that are scalable.